<!DOCTYPE html>
<html lang="en">

<head>
  <meta charset="UTF-8">
  <meta http-equiv="X-UA-Compatible" content="IE=edge">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>Document</title>
</head>
<style>
  section {
    overflow: hidden;
    width: 400px;
    height: 500px;
    border: 1px solid pink;
    position: relative;
  }

  /* 1.渐变 */
  div {
    margin: 0 auto;
    width: 800px;
    height: 500px;
    position: absolute;
    left: calc(50% - 400px);
    top: 0;
    background: radial-gradient(circle at 50% -210px, #000, #000 400px, transparent 401px, transparent);
  }

  /*2. clip-path */

  /*3.伪元素 */
</style>

<body>
  <section>
    <div></div>
  </section>

</body>

</html>